1. Field of the Invention
The present invention relates to a keypad apparatus for an input device having a backlighting function, such as a mobile phone, a telephone, a Portable Digital Assistant (PDA), a keyboard, or a calculator, and more particularly, to a back light unit for a keypad apparatus, having an easy bending property, flexibility, and optimal transmittance and preventing light leakage at an edge.
2. Description of the Related Art
A light receiving type flat panel display such as a liquid crystal display (LCD), a keypad back light unit for a telephone or a mobile phone, or a back light unit for a lighting signboard has been disclosed in Korean Patent Application Nos. 10-1997-0073783, 10-1998-0030060, 10-1999-0047621, 10-2000-0063111, 10-2001-0073468, 10-2005-0062790, and 10-2005-0062797, and Korean Utility Model Application No. 20-1996-0047524. The above key backlighting back light unit for an input device disclosed in the above inventions and utility model is of a type in which a plurality of light emitting diodes (LED) are disposed at each required part of the back of a key, and locally emit light only to each required part. Alternately, the key backlighting back light unit is of an edge light type in which a light emitting plate such as an organic light emitting diode (OLED) is provided or an LED light source is provided at an edge of a flexible thin silicon plate used as a light guide plate, and irradiates light to the required part.
The back light unit of the type in which the plurality of LEDs are directly disposed at the back of the key has a drawback that a manufacture cost increases because the LEDs are required as many as the number of the keys, and an amount of power consumption increases.
As a solution to this problem, there has been provided a method in which an LED is not provided every key but is suitably disposed in space between keys, thereby lighting two or three keys by one LED. However, this method has a serious disadvantage in which a product quality is deteriorated because a constant luminance is not provided as much as required every key, and a distribution of luminance between the keys is great. Also, this method requires still many LEDs smaller than the number of the keys.
The back light unit of the type in which a surface light emitting plate device such as the OLED in place of an LED is disposed on an upper surface of a contact switch and keys are again disposed on the upper surface thereof has drawbacks of deteriorating light efficiency of the device itself, increasing a manufacture cost of the OLED because of a great prime cost, additionally requiring a separate power supply using alternate current driving, causing a noise due to the alternate current driving, and increasing a cost for additional solutions thereto.
As a solution to the above drawbacks, there is provided an edge light type back light unit for an LCD panel including a transparent light guide plate for transmitting light and a light emitting unit disposed at an edge of the light guide plate. The edge light type back light unit is widely used because of its high efficiency and low price. However, the edge light type back light unit cannot be used as a key backlighting back light unit for an input device such as a telephone and a mobile phone because its light guide plate is of acrylic resin not almost bent. In detail, the key backlighting back light unit cannot be used for the edge light type back light unit because its keypad used for the telephone and the mobile phone should be flexible in its function and key pressing should result in pressing down of the keypad, thereby enabling a contact switch.
Thus, there is provided a method in which the light guide plate is of relatively flexible silicon in place of solid material, in order to use an edge light emitting type back light unit as the key backlighting back light unit for the input device.
However, this method causes another problem of deteriorating a product elasticity for bending and restoring the silicon light guide plate to an original state because of excessive flexibility, in case where the silicon light guide plate is manufactured as a thin film having a thickness of 1 mm or less. In particular, the silicon light guide plate is bent as mounted when there exists a bending part of the keypad such as a metal dome switch, thereby destroying a horizontal alignment of a light source and the light guide plate, and causing a loss of light incident on an end of the light guide plate. Also, there occurs a drawback that the bending of the light guide plate causes an increase of an amount of light incident at an angle of less than a critical angle for total reflection and not totally reflecting within the light guide plate, and thus causing light loss outside the light guide plate in course of light guide.
Silicon having relatively low interface energy easily forms an interface. Therefore, the silicon light guide plate is adhered to other parts upon keypad assembly, thereby making it difficult to minutely adjust and assemble the light guide plate in a regular position. The adhesion greatly occurs because the light guide plate is of a very small thickness and thus is easily bent.
In a conventional back light unit, a light source is disposed at any end of a light guide plate with a suitable light guide pattern. While incident light emitted from the light source travels within the light guide plate, light encounters the light guide pattern, effectively turns to the front, and radiates outside, thereby lighting the back of a key. Other light not encountering the light guide pattern totally reflects or transmits and disappears within the light guide plate. A large amount of light escapes from an edge of the light guide plate opposite to the light source. Thus, the conventional back light unit has a drawback that a product is greatly deteriorated in its appearance because light leaks out from an edge of the product.
In order to minimize light loss and prevent light leakage, an LCD back light unit improves light efficiency in such a manner that a reflection sheet is attached to an edge of a light guide plate at which a light source is not located, thereby enabling light to encounter and reflect from the reflection sheet and be again incident inside the light guide plate. However, a key backlighting back light unit has a great difficulty and thus has no possibility in attaching a reflection sheet to an external part of a light guide plate because the light guide plate has a very small thickness of less than 0.1 mm to 1 mm because of a product characteristic. The key backlighting back light unit is difficult to expect even the improvement of light efficiency because it cannot achieve an effect of perfect adherence to an edge. In other words, this drawback results in a loss of a large amount of light and therefore, causes a disadvantage that the number of light sources should increase by an amount of lost light to acquire a required amount of light.